Name: valence electrons, lewis symbols, and bonding
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 36 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Lewis Symbols and the Octet Rule

Il legame chimico comporta la condivisione o il trasferimento di elettroni di valenza fra due o più atomi che portano all'abbassamento dell'energia potenziale, fattore che contribuisce alla formazione del legame. Il processo di formazione del legame usando gli elettroni di valenza è spiegato dal modello di Lewis. Nel modello di Lewis, ogni elettrone di valenza di un atomo, dai blocchi s o p, è simbolizzato come un punto che circonda la sigla dell'elemento, chiamato simbolo di Lewis.

L'ossigeno, ad esempio, ha una configurazione elettronica di 1s2, 2s2, 2p4 e quindi ha sei elettroni di valenza. I primi quattro punti sono posizionati su ciascun lato del simbolo dell'ossigeno, un punto in ciascuna direzione. I restanti due elettroni di valenza sono posti sugli altri due lati attorno all'atomo.

Ogni lato è equivalente e può accogliere un massimo di due elettroni. Il numero di punti non appaiati rappresenta il numero di legami che ogni atomo di quell'elemento può formare. Gli elementi dell'ultimo gruppo, come il neon, sono gas nobili e non partecipano attivamente al legame chimico.

I gas nobili hanno elevate energie di ionizzazione e sono molto stabili, data la loro configurazione di elettroni in un guscio completamente esterno. Il simbolo di Lewis per il neon ha otto punti, due punti su ciascun lato, che rappresentano la configurazione piena degli elettroni;in altre parole, un ottetto. La regola dell'ottetto sancisce che un atomo tende a perdere, guadagnare o condividere elettroni sotto forma di legami fino a raggiungere una configurazione elettronica stabile, un ottetto.

Considerate l'anidride carbonica. Il carbonio ha quattro elettroni spaiati e l'ossigeno ne ha due. Il numero di elettroni spaiati rappresenta il numero di elettroni necessari per raggiungere un ottetto.

Dunque, il carbonio condivide due elettroni con un ossigeno, e gli altri due elettroni con l'altro ossigeno, in modo che ogni atomo possa ottenere un ottetto. L'idrogeno e l'elio sono eccezioni alla regola dell'ottetto, poiché hanno solo un orbitale s che può contenere solo 2 elettroni;quindi si dice che l'idrogeno o l'elio ottengono un duetto.

simboli di lewis e regola dell'ottetto

I legami chimici sono interazioni complesse tra due o più atomi o ioni, che riducono l'energia potenziale della molecola. Gilbert N. Lewis sviluppò un modello chiamato modello di Lewis che semplificò la rappresentazione della formazione dei legami chimici e fornì spiegazioni semplici per i legami chimici osservati nei composti più comuni.

Modello di Lewis

Il modello di Lewis descrive la formazione di legami chimici mediante la condivisione o il trasferimento di elettroni di valenza, che aiuta a raggiungere una configurazione elettronica stabile. Un legame ionico si forma quando gli elettroni vengono trasferiti tra un metallo e un non metallo, mentre un legame covalente si forma quando gli elettroni vengono condivisi tra due non metalli.

Il modello di Lewis viene utilizzato esclusivamente per descrivere le formazioni di legami senza tenere conto dei cambiamenti di energia associati alle attrazioni e repulsioni tra elettroni e nuclei sugli atomi vicini. Sebbene queste interazioni siano fondamentali per il legame chimico, la determinazione esatta dei valori è complessa. Invece, Lewis progettò disegni speciali per rappresentare i legami chimici utilizzando elettroni di valenza, chiamati simboli di Lewis.

Simboli di Lewis

I simboli di Lewis descrivono configurazioni elettroniche di valenza di atomi e ioni monoatomici. Un simbolo di Lewis è costituito da un simbolo elementare circondato da un punto per ciascuno dei suoi elettroni di valenza. Ad esempio, il sodio ha un elettrone di valenza; quindi viene disegnato un punto attorno al simbolo Na.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1055/1055_Image 1.png" style="height: 20px;" />

Per gli elementi del gruppo principale, il numero di elettroni di valenza è indicato da un numero di gruppo composto da lettere nella tavola periodica. Ad esempio, il litio (Li) appartiene al gruppo IA e ha un elettrone; il berillio (Be) è un elemento del gruppo IIA e ha due elettroni di valenza.

Ci sono eccezioni al modello di Lewis. Nell'elio, il numero dell'elettrone di valenza non è lo stesso del numero del gruppo. I metalli di transizione, i lantanidi e gli attinidi hanno gusci interni riempiti in modo incompleto; quindi non possono essere scritti in semplici simboli di punti di Lewis.

La regola dell'ottetto

Le molecole di alogeno (F_2, Br_2, I_2 e At_2) formano legami simili a quelli della molecola di cloro: un singolo legame tra gli atomi e tre coppie solitarie di elettroni per atomo. Ciò consente a ciascun atomo di alogeno di avere una configurazione elettronica di gas nobile. La tendenza degli atomi del blocco s o p a formare abbastanza legami per ottenere otto elettroni di valenza è nota come regola dell’ottetto. La regola dell'ottetto prevede le combinazioni di atomi che avranno un'energia potenziale inferiore quando si legano insieme.

Il numero di legami che un atomo può formare può spesso essere previsto dal numero di elettroni necessari per raggiungere un ottetto (otto elettroni di valenza); questo è particolarmente vero per i non metalli del secondo periodo della tavola periodica (C, N, O e F).

Gli elementi del gruppo 14 hanno quattro elettroni nel loro guscio più esterno e quindi richiedono altri quattro elettroni per raggiungere un ottetto. Questi quattro elettroni possono essere ottenuti formando quattro legami covalenti, come il carbonio nel CH_4 (metano) e il silicio nel SiH_4 (silano).

Gli elementi del gruppo 15 come l'azoto hanno cinque elettroni di valenza nel simbolo atomico di Lewis: una coppia solitaria e tre elettroni spaiati. Questi atomi formano tre legami covalenti, come in NH3 (ammoniaca).

Gli elementi del gruppo 16 come l'ossigeno e altri atomi ottengono un ottetto formando due legami covalenti, come un legame con due atomi di idrogeno in H2O (acqua).

Ci sono eccezioni alla regola dell'ottetto. Poiché l’idrogeno ha bisogno solo di due elettroni per riempire il suo guscio di valenza, costituisce un’eccezione alla regola dell’ottetto. In questo caso si dice che l'idrogeno abbia raggiunto un duetto. Anche gli elementi di transizione e gli elementi di transizione interni non seguono la regola dell'ottetto.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/7-3-lewis-symbols-and-structures">Openstax, Chemistry 2e, Chapter 7.3: Lewis Symbols and Structures</a>.

Simboli di Lewis e regola dell'ottetto

I legami chimici sono interazioni complesse tra due o più atomi o ioni, che riducono l'energia potenziale della molecola. Gilbert N. Lewis ha sviluppato un ...

Education

Didattica

JoVE Core

Chimica

Chemical Bonding: Basic Concepts

Legami chimici: concetti fondamentali

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

valence electrons, lewis symbols, and bonding

Chemical bonds form when atoms or ions interact in ways that lower the potential energy of a molecule. Gilbert N. Lewis created a simple model to show bond formation using valence electrons, the electrons in the outer shell. In this model, electrons are shared or transferred to help atoms reach a stable electron configuration.

The Lewis model helps describe two common bond types. An ionic bond forms when electrons move from a metal to a nonmetal. A covalent bond forms when two nonmetals share electrons. The model does not include the detailed energy changes from attractions and repulsions between electrons and nuclei on nearby atoms.

Lewis symbols are special drawings that show valence electrons for atoms and monatomic ions. A Lewis symbol uses the element symbol with one dot for each valence electron. For example, sodium is written with one dot because it has one valence electron. For many main group elements, the group number on the periodic table matches the number of valence electrons, such as lithium in group IA and beryllium in group IIA.

There are exceptions to this pattern. Helium does not match its group number in a simple Lewis symbol. Transition metals, lanthanides, and actinides also do not fit the Lewis symbol pattern well because they have incompletely filled inner shells. They cannot be shown with simple Lewis dot symbols.

The octet rule helps predict how many bonds many s- and p-block atoms will form. These atoms often form enough bonds to reach eight valence electrons, which gives them a noble gas electron configuration. This idea is especially useful for second-period nonmetals such as carbon, nitrogen, oxygen, and fluorine. It can help predict which combinations of atoms are likely to have lower potential energy when they bond.

Group 14 elements have four valence electrons and need four more to reach an octet. They can do this by forming four covalent bonds, as carbon does in methane, CH4, and silicon does in silane, SiH4. Group 15 elements such as nitrogen have five valence electrons, shown as one lone pair and three unpaired electrons, so they usually form three covalent bonds, as in ammonia, NH3. Group 16 elements such as oxygen form two covalent bonds to complete an octet, as in water, H2O.

Not every atom follows the octet rule. Hydrogen is an exception because it only needs two electrons in its valence shell. When hydrogen reaches this state, it has a duet, not an octet. Transition elements and inner transition elements also do not follow the octet rule.

Valence Electrons, Lewis Symbols, and Bonding

valence-electrons-lewis-symbols-and-bonding

types of chemical bonds

Chemical Bonding and Bond Properties

Chemical bonding explains how atoms and ions join to form molecules and compounds. American chemist Gilbert N. Lewis pioneered chemical bonding theory with the Lewis model. This model helps explain why bonds form, why some are stronger than others, and why compounds have fixed formulas like H2O instead of H4O. Ionic bonding involves ions, which are atoms or molecules with an electrical charge. A cation is a positive ion formed when a neutral atom loses one or more electrons from its valence...

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Lattice energy helps explain why ionic compounds are stable. It measures the strength of attraction between positive and negative ions in a crystal. In one common definition, lattice energy is the energy required to separate one mole of an ionic solid into gaseous ions, so it has a positive value. Another definition uses the opposite sign and describes the energy released when gaseous ions form a lattice. When checking data, it is important to confirm which convention is being used. In either...

lewis symbols and the octet rule

Chemical bonds form when atoms or ions interact in ways that lower the potential energy of a molecule. Gilbert N. Lewis created a simple model to show bond formation using valence electrons, the electrons in the outer shell. In this model, electrons are shared or transferred to help atoms reach a stable electron configuration. The Lewis model helps describe two common bond types. An ionic bond forms when electrons move from a metal to a nonmetal. A covalent bond forms when two nonmetals share...

electronegativity

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Ionic Compounds and Crystal Lattices

Ionic compounds form when positive and negative ions attract each other. A cation is a positive ion, and it forms when an atom loses one or more valence electrons. An anion is a negative ion, and it forms when an atom gains one or more valence electrons. Compounds made of these ions are called ionic compounds, or salts, and the force between them is an ionic bond. The properties of ionic solids help show how strong this attraction is. They usually form rigid, brittle crystals and have high...

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Lattice energy shows how strongly ions hold an ionic solid together. It is the energy required to separate one mole of a solid into its gaseous ions. For sodium chloride, this process requires 769 kJ per mole. The opposite convention may also be used, where lattice energy is the energy released when gaseous ions form the solid. In either case, a larger magnitude means a more stable ionic compound. For ionic crystals, lattice energy follows a pattern based on charge and ion size. The...

covalent bonding and lewis structures

Shared Electrons in Covalent Molecules

Covalent molecules form when atoms share electrons instead of transferring them. This bonding usually happens between nonmetal atoms with similar tendencies to attract electrons. Because the atoms pull on the electrons in a shared way, the bond is held together by mutual attraction. Covalent compounds often have physical properties that differ from ionic compounds. The attraction between neutral molecules is weaker than the attraction between charged ions. As a result, many covalent compounds...

bond polarity, dipole moment, and percent ionic character

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How Bond Polarity Shapes Molecular Dipoles

Bond polarity depends on electronegativity, the ability of an atom to attract shared electrons. The absolute value of the electronegativity difference, or ΔEN, gives a rough guide to bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is larger, the bond may be polar covalent or ionic. Examples show how this works in real compounds. H–H has a ΔEN of 0, so it is nonpolar. H–Cl has a ΔEN of 0.9, so it is polar covalent. Na–Cl has a ΔEN of 2.1, so it...

lewis structures of molecular compounds and polyatomic ions

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Drawing Lewis Structures with Valence Electron Counts

Drawing Lewis structures starts with counting valence electrons and building the atom arrangement step by step. Valence electrons are the outer-shell electrons that help show how atoms bond. For ions, a positive charge means one electron is removed for each plus sign, and a negative charge means one electron is added for each minus sign. The next step is to sketch the skeleton structure. The least electronegative atom usually goes in the center, while hydrogen is almost never central. Fluorine...

resonance

Resonance Forms in Lewis Structures

Resonance forms in Lewis structures help explain molecules and ions that cannot be shown correctly with just one drawing. In nitrite ion, NO2−, the Lewis structure can be drawn in two ways. The N-O and N=O bonds switch positions in the two drawings, but the atoms stay in the same arrangement. If nitrite really had one single bond and one double bond, the two bond lengths should be different. A double bond is shorter and stronger than a single bond between the same atoms. Experiments show that...

formal charges

Lewis Structures with Formal Charge

Lewis structures can sometimes show more than one reasonable arrangement for a molecule or polyatomic ion. Formal charge helps chemists choose the best Lewis structure when that happens. It is a useful way to compare possible structures in chemistry. Formal charge is the hypothetical charge an atom would have if bonding electrons were shared evenly between the atoms. It can also be found by starting with the valence electrons of the neutral atom, subtracting the nonbonding electrons, and then...

exceptions to the octet rule

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Some covalent molecules do not follow the octet rule in their Lewis structures. Their central atoms can fall into three groups: odd-electron molecules, electron-deficient molecules, and hypervalent molecules. These cases show why a Lewis structure may not always give every atom eight valence electrons. Odd-electron molecules have an odd number of valence electrons, so one electron remains unpaired. Molecules with an odd number of electrons are called radicals. Nitric oxide, NO, is one example,...

bond energies and bond lengths

How Bond Energy Predicts Reaction Heat

Bond energy helps explain why molecules stay together and how much heat a reaction may release or absorb. A covalent bond is stronger when more energy is needed to separate the atoms. The energy needed to break one specific covalent bond in one mole of gaseous molecules is called bond energy, or bond dissociation energy. For a diatomic molecule, bond energy is the standard enthalpy change for an endothermic reaction that breaks that bond. The H-H bond is a common example. Its bond energy is...

bonding in metals

Electron Sea Model in Metals

Electron sea model explains how metallic bonding holds metals together. In this model, most metal atoms do not have enough valence electrons to form ionic or covalent bonds in the usual way. Their valence electrons are held loosely because metals have low electronegativity, and their ionization energy is low, so electrons are removed easily. When a metal atom loses these outer electrons, it becomes a positively charged metal ion. The freed electrons form a delocalized electron cloud around...

key terms and concepts

Learn how valence electrons, Lewis symbols, and the octet rule help predict ionic and covalent bonding in common high school chemistry examples.

Valence Electrons, Lewis Symbols, and Bonding | JoVE K12

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JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

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Ohio’s Learning Standards and Model Curriculum

Physical Science

Study Of Matter

Bonding and compounds

Structure And Properties Of Matter

Chemical bonding

Physical Geology

Minerals

Chemical bonding (ionic, covalent, metallic)

Tennessee Academic Standards

High School

Matter and Its Interactions

CHEM1.PS1.12

CHEM1.PS1.14

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CHEM1.PS2.1

CHEM1.PS2.2

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CH.3

CH.3.d

PS.3

PS.3.c

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PH.9

PH.9.e

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HS.P1

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New Jersey Student Learning Standards for Science

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Alabama Course of Study Science

9-12.C.2.2.a

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New York State Science Learning Standards

HS. Structure and Properties of Matter

HS-PS1-3.

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HS-PS1-2.

Georgia Standards of Excellence

SC2.

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Valence Electrons, Lewis Symbols, and Bonding

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